Electrophoretically fractionated mononucleosomes are heterogeneous. Five classes exist for calf thymus chromatin, and three classes exist for cultured mouse cell chromatin. We have partially characterized the precursor-product relationships between these components, their DNA lengths, and their overall histone compositions. Of major interest are the findings that certain classes of the monomers originate from different DNA repeats. We wish to further study the various monomers with regard to histone stoichiometry, histone modifications, and nonhistone proteins. These goals will be fullfilled by use of isotope labeling experiments with cultured cells. Different mononucleosome classes will be extracted from gels, and their proteins will be resolved by acid-urea gels, triton acid-urea gels and SDS gels. Gels will be sliced and counted; the histone:DNA mass ratios and stoichiometry of histones will be calculated. Chromatin will also be crossed-linked with (bis)imidoesters, the resulting material fractionated by native gel electrophoresis, and 2nd dimension gels run to count the limit cross-linked number of histones per particle. BIBLIOGRAPHIC REFERENCES: Garrard, W.T., Todd, R.D., Boatwright, D.T., and Albright, S.C. (1976) Stoichiometry of Histones in Chromatin Subunits. Tenth International Congress of Biochemistry, Abstr. 02-3-178. Garrard, W.T., and Hancock R. (1977) Preparation of Chromatin from Animal Tissues and Cultured Cells in Methods of Cell Biology, 17, ed. Stein, Stein, and Kleinsmith. Academic Press, New York.